Description of the Prior Art
Particle fallout generally occurs when large particles, typically greater than 5-10 microns in diameter, are deposited on the surfaces of processing equipment or the materials being processed. Such particles take the form of skin flakes, threads from clothing, or debris from operating machinery that may settle out, by way of example. Particle fallout may adversely affect contamination-sensitive processes and products. For example, in the assembly of sensitive components for satellites, excessive particle fallout can lead to failures after launch. Also, particles of this type can adversely affect manufacturing processes and the yield of products, such as printed circuit boards and hybrids.
Particle detectors are used to monitor levels of particulates that are present in a manufacturing environment, such as a semiconductor wafer processing system. The detected particulate level or count is used to localize and control the source of problems that occur during manufacturing processes in order to obtain optimum yields in the production of contamination-sensitive devices such as integrated circuits.
Optical particle counters are described in the textbook "Aerosol Technology" by William C. Hinds, published by John Wiley & Sons, 1982, at pages 341-344:
"As shown in FIG. 16.18 of this reference, air is drawn through a sensitive volume. This air carries particles that can be detected using light scattering as they pass through the sensitive volume. However, large particles do not remain effectively suspended in air and cannot efficiently negotiate the long length of tubing required to bring the air from the point in the environment to be sampled to the sensitive volume in the optical particle counter. As a consequence, the efficiency of this instrument becomes very low for particles larger in size than 10 microns. This is illustrated in Table 16.3 of the same reference, which shows that the upper limit of the effective size range is around 10 microns."
One type of particle detector that may be used for detection of spurious particles is disclosed in copending application Ser. No. 06/907,776, which issued Apr. 19, 1988, as U.S. Pat. No. 4,739,177, and is incorporated hereby by reference. The referenced patent discloses a particle detector for semiconductor wafer processing equipment wherein a light beam is employed that is reflected multiple times between mirrors to form a light sheet. Particles traversing the light sheet cause light to scatter, and the scattered light is detected by photosensing means.
In the particle flux monitor described in the referenced patent, particles having a diameter up to several millimeters are detected in real time. However, many fallout particles do not fall or drop straight down but are carried by air currents as they proceed to fall. Therefore, air currents that occur close to the sensor device can draw particles, such as skin flakes, through the sensor that do not necessarily fall onto a surface of the unit in production or onto contamination-sensitive manufacturing equipment. As a result, the sensor will count many more particles than those that actually fall out and impinge on the contamination-sensitive surface.
In one approach, witness plate methods are applied to monitor particle fallout. A flat plate is placed at a critical location to collect fallout over a period of time. The plate is then viewed under a microscope and the particles are counted. This technique is not implemented in real time and in effect indicates only when a fallout generating event has previously occurred. Furthermore, the technique is labor intensive, subject to human error and difficult to automate.
It is highly desirable to be able to rapidly find the active sources of the particle fallout to allow steps to be taken to correct the problem and control fallout levels before serious damage occurs.